Large lipid-insoluble drug molecules resembling amino acids, peptides, or glucose require special carrier proteins for diffusion across the cell membrane. The transport can occur without an energy input—known as facilitated diffusion or at the expense of energy—termed active transport. Facilitated diffusion is a passive process involving SLC transporters that bind to the drug, undergo conformational changes, and release the drug on the other side. In active transport, the drug moves against its concentration gradient. It can be further categorized based on the driving force. In primary active transport, ABC transporters use energy from ATP hydrolysis to mediate the unidirectional efflux of the drugs. In secondary active transport, SLC transporters move drugs against their concentration gradient. This is driven by the electrochemical potential resulting from the movement of another solute along its gradient. Symport is when the two move in the same direction; antiport is when they move in opposite directions.